Some of the plastic that the city asks residents to put out for recycling is actually going to landfills.

Mayor Michael Bloomberg told New Yorkers seven months ago, "If it's a rigid plastic—any rigid plastic—recycle it." But not all of the rigid plastic city residents have been bagging with their metal cans, beer bottles and juice boxes since that announcement is being recycled.

Nearly all of the rigid No. 6 plastic—commonly used in cups, plates and other objects—delivered by the city's trucks to its recycling vendor, Sims Metal Management, has been shipped to dumps because Sims can't sell it.

"Right now that is going to landfill," said Tom Outerbridge, general manager for Sims.

Last April 22 was Mr. Bloomberg's 12th and final Earth Day as mayor, and he doubtless wanted to announce something significant about recycling that week. But the expansion of recycling program to include all rigid plastic was ahead of its time—at least by a few months.

In December, Sims will open a Brooklyn facility that uses optical-sorting technology to automatically separate different kinds of plastic.

"With this plant open, I'm going to be able to start producing straight, segregated loads of [No. 6] polystyrene products and I'm told there will be some people who will buy it," Mr. Outerbridge said.

He acknowledged that it would have been better for Sims and the city to wait until the sorting technology was ready before expanding curbside collection of plastic.

"Ideally we would have waited a few more months until this place was fully up and running," he said of the new plant, which he said could open in early December. "But we're excited about building some momentum for the recycling program."

Roger Bernstein, vice president of state affairs for the American Chemistry Council, an industry group, said the Bloomberg administration was being dishonest by accepting material in its recycling program that are not actually recycled. "They are actually greenwashing the consumer," he said.

A spokesman for the mayor said, "We attempted to time the expansion of all rigid plastics with the opening of the new Sims recycling facility in Brooklyn, which was slightly delayed. Notwithstanding that, we implemented the expansion for almost all rigid plastics and, as Sims has said, when the facility opens in the next couple of weeks, No. 6 rigid plastics—which represent 0.2% of the city's waste stream—will be sorted and recycled as well."

Makers of plastic-foam food containers, who are facing a ban on their products, are citing the city's failure to recycle some rigid plastic as a reason to accept their offer to subsidize a recycling program for both foam and rigid No. 6. Bloomberg officials say the logistics are unworkable and Sims, which has the final word on what it accepts, is not enthusiastic about the manufacturers' offer.

Although most of the rigid No. 6 plastic collected since Earth Day has been sent to landfills, there has been an environmental benefit to the expansion: Sims has found markets for some of the other plastics added to the curbside program, which previously only accepted bottles, cans and jugs.

"On a regular basis we sell the [No. 5] polypropylene, and the bulky plastics—the crates and hampers," which are generally made of Nos. 1, 2, and 5 plastic, Mr. Outerbridge said. "And we sell No. 4 containers, low-density polyethylene." That's the resin used mostly in bags, although bags themselves are not accepted in the curbside program. (New Yorkers often attempt to recycle them anyway, causing major problems for Sims because the bags get caught in the company's machinery. "We get a huge amount of bags inside of bags inside of bags," Mr. Outerbridge said. "We get those giant bags that people get their mattresses in, too. That's a nightmare." The city's sanitation crews are supposed to leave that material on the curb, but some end up in trucks.)

City officials were likely aware that rigid No. 6 plastic might not get recycled until Sims could sort it more efficiently, but excluding it from last April's expansion would have confused New Yorkers who already struggle to remember what to recycle. Rather than ask them to find the tiny number inside the recycling logo on most plastics, the mayor had them bag all rigid plastics with their metals and juice boxes.

"We support the objective of getting more material into the recycle bin and making it easier for the public," Mr. Outerbridge said. "And we accept that we're going to end up with more material that we don't have a home for."

The cost of sending waste to landfills is currently about $85 a ton. That number is expected to rise in the years to come, as landfill space becomes more scarce and transportation costs increase. If Sims continues to struggle to sell No. 6 plastic, it could ask the city to take action, such as encourage the use of PET, a more marketable type of plastic.

"I'm not 100% confident that we are going to have a reliable market" for rigid No. 6, Mr. Outerbridge said." If it comes to that, he said, "I would go to the City Council and say, 'Hey, we tried, and I don't like to be the person to say get rid of this material, but no one will buy this material on an ongoing basis, [so] we would like the city to advance some legislation to transition from [rigid] polystyrene to PET.' "

Eric A. Goldstein, the Natural Resources Defense Council's New York City environment director, said the market for No. 6 plastic could develop in the years to come. "When the city started its recycling program [in 1989], the markets for 1 and 2 were weak," he said. "Now they are very strong markets."